Hybrid high rate anaerobic treatment process

ABSTRACT

An anaerobic reactor for treatment of waste water comprises a primary reactor or zone of the suspended growth type and a secondary reactor or zone of the fixed filter bed type disposed directly above and in liquid contact with the primary reactor. Liquid to be treated is fed continuously into the primary reactor and moves upwardly through the primary reactor, then through the filter bed and out via an outlet above the filter bed. Gas generated in the primary reactor moves upwardly through the liquids and through the filter bed, to cause vertical mixing throughout the zones and to keep the filter bed from clogging.

FIELD OF THE INVENTION

This invention relates to high rate anaerobic treatment technology andmore specifically to a hybrid anaerobic treatment process and apparatus.

Anaerobic waste water treatment processes offer several advantages overaerobic treatment processes in that they have lower energy requirementsand can greatly reduce the quantities of sludge generated for ultimatedisposal. Moreover, one of the end products of anaerobic treatment is acombustible gas, which can be used as an energy source.

BACKGROUND OF THE INVENTION

The anaerobic digestion process is a two stage process involvinghydrolyzing, acid forming, and methane forming bacteria. In the firststage, the hydrolyzing and acid forming bacteria convert volatile solidmaterials to soluble compounds and then to various volatile fatty acidssuch as acetic acid, generating carbon dioxide and hydrogen in theprocess. In the second stage, the methane forming bacteria convert thevolatile fatty acids to methane and carbon dioxide, and also combinecarbon dioxide with hydrogen to form methane and water.

With many wastes, the waste components are largely soluble, and theconversion of volatile solids material to soluble compounds is notcritical.

The overall reaction is as follows: ##STR1## The proton acceptor may beNO₃ --, which is reduced to NO₂ --, N₂ O, or N₂. If SO₄ ═ is present, itwill be reduced to H₂ S.

Depending on the waste composition, the rate limiting step in thereaction may be either volatile solids conversion or methane production.Many industrial and food processing wastes contain significant solublecompounds but relatively less volatile solids materials. The ratelimiting step in the reaction of these wastes is the methane productionstep. The methane forming bacteria are sensitive to temperature, food,and pH conditions so that these conditions should be optimized so as toincrease the overall rate of the process. Thus, anaerobic digestion isusually carried out at 25° C.-40° C., preferably at about 35° C.Anaerobic digestion can also occur from 15° C. to 60° C. The pH of theprocess at steady state is normally around 6.8, although it can rangefrom 6.0 to 8.0. Large changes in system temperature or pH candeactivate the methane-forming bacteria, but if the system returns toideal conditions these bacteria will become active again.

BRIEF REFERENCE TO THE PRIOR ART

Several processes and reactor types are known for anaerobic waste watertreatments. One of these is the anaerobic filter process. In thisprocess there is provided a fixed supporting medium such as rocks,ceramic pieces, wood pieces, plastic pieces or the like, on which thesludge containing the microorganisms is grown, and in the interstitialspaces of which the sludge is grown, and the waste water is passedthrough the medium in the manner of a filter. The contact time can beadjusted by adjustment of the depth of the bed and the rate of flow ofthe waste water therethrough. Whilst the process is efficient in that itgives high COD reductions when operated at high rates, it suffers fromthe disadvantage that the bed tends to become filled with solids after atime. This occurs due to sludge growth, inorganic precipitation and highsolids contents in the waste water. When the filter bed accumulatesexcess solids, it loses efficiency, the contact time of the waste waterwith the sludge in the filter bed during a continuous flow process beingundesirably reduced.

Another known process is the anaerobic contact process in which sludgecontaining the microorganisms is suspended in a liquid medium to whichthe waste water is fed, solids/liquid separation being undertaken in aseparate vessel and the solids, which include the sludge, beingrecycled. This process tends to suffer from the problem of inefficientsolids/liquid separation in the second vessel, resulting in solidslosses, especially when conducted at high rates.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a novel high rateanaerobic waste water treatment process and apparatus which reduces orovercomes at least one of the disadvantages of the known systems.

The present invention provides a process and apparatus which combinesthe advantageous features of the anaerobic filter process and theanaerobic contact process and at the same time substantially reduces thedisadvantages of both. According to the invention, the waste water isfirst fed to a lower zone or primary reactor containing suspendedanaerobic sludge with the appropriate microorganisms. Anaerobictreatment commences in this lower zone, with generation of gases, in themanner of the contact process. Above the primary reactor, which iseffectively a suspended growth zone, there is provided an anaerobicfilter bed, supported on a grating or similar liquid permeable support.The waste water under treatment passes upwardly from the primary reactorthrough the filter bed, where it contacts further quantities ofanaerobic sludge for further treatment. The treated water then exitsfrom above the filter bed.

Thus, the gases generated in the suspended growth zone rise up throughthe liquid therein, to the filter bed, and pass through the filter bed,in a turbulent manner thereby counteracting the tendency of the filterbed to become filled with solids. Moreover, the rising gases generatedin both the suspended growth zone and the filter bed zone provide avertical mixing effect, extending through the primary reactor orsuspended growth zone and up into the filter bed (the secondaryreactor). This vertical mixing tends to reduce excesses of pHdiscrepancies between the two zones, and promote stable, steady stateconditions. The vertical mixing also induces a downward flow of liquidin some areas, cuasing the return of solids from the secondary reactorto the primary reactor. If, as commonly happens, the gas bubblesgenerated in the primary reactor attach themselves to solid particlestherein, e.g. the sludge particles active therein, they will cause theseto rise also. However, the filter bed, or the grating upon which it issupported, in the secondary reactor acts as an obstruction to freesolids passage, so that the gas bubbles become detached from the solidsat the impact and the solids drop back into the primary reactor.Moreover, some of the naturally buoyant solids borne along by the upflowof the liquid in continuous steady state operation of the process willbe retained in the secondary reactor or filter bed by entrapment withother sludge masses in the interstitial spaces between the filter mediapieces. Because of the vertical mixing, these solids can return to theprimary reactor or suspended growth zone. These features reduce thecommon problem encountered with contact reactors, of excess solidslosses to the effluent.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The microorganisms employed in the process of the invention arepreferably the same as those commonly employed in anaerobic sludgedigestion processes. Both stages of the process, namely formation ofsolubles and acids followed by methane formation, take place in bothzones, the suspended growth zone or primary reactor and the filter bedor secondary reactor. Thus the sludges in both zones contain the samemicroorganisms, inevitably, because of the free liquid communicationbetween them, establishing equilibrium or at least steady stateconditions. Non-attached, suspended growth types are naturally favouredin the suspended growth zone (primary reactor) whereas, in the filterbed, more growth of attached organisms is naturally promoted, but thesame functional organisms are present in both zones. It is a furtheradvantageous feature of the process of the present invention that eachreactor zone effectively provides a reservoir of active microorganismsfor the other. Thus, in the event of a sudden change in the nature ofthe incoming waste water into the primary reactor due to upstreamvariations or errors, which might initially kill some or all of thedesired microorganisms therein, the stock will be replenished and steadystate resumed by supply of organisms from the filter bed. Similarly, inthe event of a sudden surge of liquid through the filter bed, or otherexceptional process conditions having the effect of washing out orkilling substantial numbers of the microorganisms from the filter bed,it can be re-seeded from the suspended growth zone or primary reactor.

Another advantageous feature of the invention is the ability of theapparatus to operate efficiently for long periods of time beforecleaning or purging of solids is required. An anaerobic filter processruns efficiently in the early stages of operation and less efficientlyas the filter bed accumulates excess solids, whereas an anaerobiccontact process runs less efficiently in its early operation andimproves in operation as solids accumulate. By combining the twoprocesses, the present invention is efficient during both the early andlater stages of operation.

In the preferred apparatus according to the invention, the primary andsecondary reactors or zones are provided vertically above one another,e.g. as two parts of a cylindrical tank separated only by the gratingsupporting the filter bed. Liquid contact between the zones is providedover substantially the whole mutual surface areas thereof. The volume ofthe primary reactor or suspended growth zone should be at least as largeas, and preferably larger than, the volume of the secondary reactor, toallow for adequate relative residence times in the two zones for thedesired reactions, gas production, mixing and solids movement to occur.

Preferably also, agitation and liquid distribution means are provided inthe primary reactor, to supplement the agitation and mixing caused bythe rising gas flow through the two zones. Conveniently, the agitationmeans comprises a system of inlet and outlet ports, spaced around theperiphery of the primary reactor, through which liquid can be circulatedand recirculated to effect agitation. Preferably also, the waste will beintroduced to the reactor through the inlet ports, in a sequentialmanner, to provide the effective distribution of flow to the primaryreactor. With smaller reactors, the number of inlet and outlet ports canbe reduced, even to a single port.

The temperature and pH conditions of operation of the process of thepresent invention are generally in accordance with known anaerobictreatment processes, e.g. temperatures of liquid undergoing treatment offrom about 15° C.-60° C., and pH of from about 6-8.

BRIEF REFERENCE TO THE DRAWINGS

FIG. 1 is a vertical cross-sectional view of a reactor apparatusaccording to the preferred embodiment of the invention;

FIG. 2 is a horizontal cross-sectional view of the reactor apparatus ofFIG. 1, taken along the line 2--2 of FIG. 1.

SPECIFIC DESCRIPTION OF THE PREFERRED EMBODIMENT

As can be seen in FIG. 1, the reactor apparatus has a generallycylindrical hollow body 12 and a conical bottom end 14 terminating in anapex 16. The apparatus 12 comprises upper and lower reactors or zones,18 and 20 respectively, separated by a grating 22 on which is supported,in the upper zone 18, a filter bed 24 of thermoplastic rings on whichanaerobic sludge is attached and grown. The upper zone 18 includes thefilter bed 24 and thus constitutes a secondary reactor of the anaerobicfixed bed type.

The lower reactor 20 is of greater volume than the upper secondaryreactor or zone 18, and contains waste water to be treated, andanaerobic sludge suspended therein. The lower reactor or zone 20 thusconstitutes a primary reactor of the anaerobic contact type, in liquidcommunication over substantially its entire surface area with the upperreaction zone 18, at the grating 22. The primary reactor 20 defines asuspended growth zone in which growth of suspended sludge particles canoccur. It includes a plurality of waste water inlet pipes 25 distributedaround the periphery of the reactor 20 and near the bottom thereof (seealso FIG. 2). The ends of pipes 25 within the reactor 20 are inclineddownwardly as shown but can be horizontal to induce a lateral flow.There is also included a draining and seeding pipeline 26, extending tothe apex 16, to allow seed bacteria to be fed into the reactor or excesssolids removed, when required.

Lower reactor 20 is also provided, at its upper portion just below thegrating 22, with a plurality of liquid withdrawal pipes 28, the ends ofwhich are spaced around the inner periphery of the reactor 20. Therelative disposition of the ends of inlet pipes 25 and withdrawal pipes28 is generally as shown in FIG. 2, namely, an inlet pipe end isdiametrically opposite to a withdrawal pipe end. By means of appropriatevalving and interconnection, an inlet pipe 25 can be connected to itsdiametrically opposed withdrawal pipe 28 and waste water recirculated bypumping therebetween, so that the pipe system constitutes an agitationmeans for the reactor.

The upper, secondary reactor or zone 18 is disposed vertically above theprimary reactor 20, and contains the filter bed 24. Above the filter bed24 is provided an effluent outlet 30 in the form of a horizontal pipingarrangement having points, such as upturned pipe elbows or otherfittings, where the liquid can overflow into the piping and therebyleave the apparatus. Above outlet 30 is an emergency overflow 32, oftenrequired for safety purposes, and an uppermost gas outlet 34. Both theupper reactor 18, and the lower reactor 20 are provided with appropriatesensing and sampling means 36 shown in FIG. 1 in broken line.

In operation, waste water to be treated, at a suitable temperature suchas 35° C., is pumped continuously into the primary reactor zone 20through inlet pipes 25, encountering and reacting with anaerobic sludge,so that hydrogen, carbon dioxide and methane gas bubbles are generatedin zone 20. The liquid flows upwardly through grating 22 and filter bed24 in upper secondary reactor zone 18, encountering and reacting withmore anaerobic sludge in the filter bed 24. Treated liquid then exitsvia effluent outlet 30, in a suitably cleansed and purified condition.

During the treatment, gas generated in lower zone 20 rises, and passesinto and through filter bed 24 and out to collection through outlet 34.As it rises, it causes a degree of vertical agitation throughout bothzones of the apparatus, thereby assisting in the maintenance of uniformand steady state conditions therethrough, similar pH values and reactantconcentrations at various levels, replenishment of microorganisms in thelower zone, etc. It also assists, during its passage through filter bed24, in dislodgement of excess deposited solids therein, so as to helpprevent clogging of the filter bed. Solid particles which rise from thelower zone 20, e.g. in association with rising gas bubbles, areeffectively prevented from passing through the filter bed, and perhapscontributing to the clogging thereof, by the grating 22 and the media inthe filter bed. On impact with the grating and media, the gas bubblesare detached from the solid particles to continue upwards through thefilter bed 24, whilst the residual solid particles have lost theirbuoyancy and hence sink.

Agitation in the lower zone 20 is supplemented from time to time by useof the withdrawal pipes 28 and inlet pipes 25. Waste water from withinzone 20 is recycled by suction from a withdrawal pipe 28 to adiametrically opposed inlet pipe 25, to cause agitation in the zone 20.Since the inlet pipes are disposed vertically below the withdrawalpipes, this also contributes to the vertical mixing effect and downwardreturn of solids and liquid desirable to keep the filter bed 24unplugged and to sustain steady state conditions. Incoming feed can beintroduced with the recycled flow to the inlet pipes, or separately tothe bottom of zone 20.

The apparatus and process as illustrated operates efficiently and overextended periods of time to reduce BOD and COD in waste water from awide variety of sources, including municipal and industrial wastes,especially from plants handling organic compounds such as foodprocessors. Such waste waters are commonly already at a suitabletemperature, following previous subjection to pressure cooking,blanching, thermal conditioning, vacuum filtration and the like.Typically, an apparatus according to the present invention has a totalvolumentary capacity of 7000 cubic meters, and employs residence timesfor waste water therein of 48 hours in the primary reactor and 16 hoursin the secondary reactor.

Variations in the design and operation of the apparatus and process fromthat specifically described are of course possible and are within thescope of the present invention. For example, the filter medium of thefilter bed used in the secondary reactor can be any of the stable inertsubstances known as suitable in standard anaerobic fixed filter beds.The dimensions, numbers and shapes of the reactor components can bevaried. The reactor could have a flat bottom or could have hoppers inthe bottom instead of having a conical bottom end 14. Also, the relativedisposition of the ends of inlet pipes 25 and withdrawal pipes 28 can bechanged.

The operation of the process of the preferred embodiment of theinvention is illustrated in the following specific example.

EXAMPLE

An anaerobic hybrid reactor was constructed according to the design ofthe reactor of the preferred embodiment of the invention, as illustratedin the accompanying drawing. The filter bed packing material wasthermoplastic Raschig rings.

The dimensions of this reactor are given in Table I.

                  TABLE I    ______________________________________    Reactor Dimensions    ______________________________________    Volume of primary     10.1 L    Volume of secondary    6.6 L    Total Reactor Volume  16.7 L    Filter Section Initial Void Volume                           5.9 L    ______________________________________

The waste water fed to the reactor was a thermal conditioning liquor,produced as a by-product of sludge processing, and having thecharacteristics given in Table II below.

It was fed into the reactor at an average rate of 13L/day, cooled totemperature of about 35° C. This rate of feed gave a residence time inthe primary, lower reactor of about 18.6 hrs, and a residence time inthe upper, secondary reactor of about 10.9 hrs.

                  TABLE II    ______________________________________    Waste Water Composition                  Concentration (mg · L.sup.-1)    Parameter       Means     90%*    ______________________________________    COD             11,000    15,500    COD-filtered    10,700    15,000    BOD             5,500     7,500    BOD-filtered    5,300     7,000    TKN               875     1,110    NH.sub.4 --N      300       500    Total P           65        88    Suspended Solids                      150       520    Volatile Acids  1,430     2,200    pH              4-5    Alkalinity       1414    ______________________________________     *90% of observations less than value indicated.

During periods of the experiment, the feed COD and feed BOD- filteredaveraged as low as 5000 mg/L and 2,200 mg/L respectively.

The reactor conditions after a period of operation of several months aresummarised in Table III.

                  TABLE III    ______________________________________    Reactor Conditions    ______________________________________    feed COD        5000 mg/L    feed BOD.sub.5 - filtered                    2200 mg/L    COD load        65 g/day    reactor loading rate                    4.1 g COD/per liter                    of initial void volume per day    reactor loading rate                    11 g COD/per liter    (based on filter only)                    of initial void volume per day    effluent BOD.sub.5 - filtered                    200 mg/L    removal of BOD.sub.5 - filtered                    91%    pH at steady state                    7.3-8.0    ______________________________________

From Table III, it can be seen that the hybrid reactor performed well interms of BOD₅ removal. After operating continuously for very extendedperiods of time, there was no sign of plugging of the filter bed.

We claim:
 1. A process for effecting anaerobic biological treatment ofwaste water with high solids content, said process comprising:contactingwaste water with anaerobic sludge having appropriate live microorganismstherein, in a first, lower reactor zone, to effect digestion of wastproducts therein and generation of gaseous by-products; moving saidwaste water and said gaseous by-products upwardly into a second, upperreactor zone disposed vertically above the first, lower reactor zone,said upper zone comprising a filter bed; effecting dislodgement ofexcess deposited solids. in said filter bed and inhibiting clogging ofsaid filter bed by passing the waste water and said gaseous by-productsupwardly through said filter bed, said filter bed having anaerobicsludge with live microorganisms therein or thereon so that the wastewater contacts the anaerobic sludge as it passes upwardly through thefilter bed to effect further digestion of waste products therein andgeneration of further gaseous by-products and so that said gaseousby-products and said further gaseous by-products pass through saidfilter bed in a turbulent manner to obtain a vertical mixing effect,said mixing effect extending through said lower reactor zone and saidupper reactor zone; and recovering treated waste water from above thefilter bed, wherein said gaseous by-products and said further gaseousby-products are removed from the process only above the filter bed andafter having passed through the filter bed in the upper reactor zonefrom at least one exit port.
 2. The process of claim 1 wherein agitationof liquid in the lower reactor zone is effected during treatmenttherein.
 3. The process of claim 2 wherein said agitation is effected bywithdrawal of liquid from one location in the lower reactor zone andconcurrent return of said liquid at another location in the lowerreactor zone.
 4. The process of claim 2 wherein said agitation iseffected by concurrent withdrawal and return of liquid from and to saidlower reactor from respective withdrawal locations and return locations,each said withdrawal location being substantially diametrically opposedto a respective return location.
 5. The process of claim 1 wherein thetreatment process is effected at a temperature of from about 15° C. toabout 60° C.
 6. The process of claim 5 wherein the pH of the waste waterundergoing treatment at steady state is from about 6.0 to about 8.0. 7.The process of claim 1, wherein gas generated in the lower reactor zoneand gas generated in the upper reactor zone are collected together fromsaid at least one exit port only above said filter bed in the upperreactor zone.